The present disclosure relates to marine electromagnetic surveying and, more particularly, to systems and methods of determining subsurface resistivity in which processing and inversion are combined into a single step.
One type of marine geophysical surveying includes marine electromagnetic surveying, in which geophysical data may be collected or acquired. Marine electromagnetic surveying is used, among other purposes, to infer spatial distribution of electrical subsurface resistivity of formations below the bottom of a body of water, such as a lake or ocean. The spatial distribution of subsurface resistivity is used to assist in determining the presence of hydrocarbon-bearing formations in the subsurface, potentially resulting in cost saving by better targeted drilling operations. Marine electromagnetic surveying generally includes inducing an electromagnetic field in the subsurface formations (e.g., by injecting electrical current in the water), and measuring one or more parameters related to a response of the subsurface formations to the induced electromagnetic field, the response referred to herein as an “electromagnetic earth response.” The current may diffuse through the sea water and subsurface formation with an electric potential difference caused by the current measured at some distance away from the electromagnetic source. The resulting electric potential difference may be measured using electromagnetic sensors, for example, that may be distributed on one or more streamers towed by survey vessel, located on one or more ocean bottom cables, or fixed at nodes at or near at a water bottom. The sensitivity of the potential difference to the subsurface resistivity may depend on the sensor offsets and the source signal frequencies.
Data may be collected from the marine electromagnetic survey to obtain information about the subsurface resistivities. The goal is typically to determine the subsurface resistivities as accurately as possible. Determination of the subsurface resistivities using the collected data commonly includes two major steps, commonly referred to as processing and inversion. Processing may be focused on providing good estimates of electromagnetic earth responses while suppressing ambient electromagnetic noise. Inversion may use the estimates of electromagnetic earth responses provided by the processing to search for a distribution of subsurface resistivities that may best fit the electromagnetic earth responses. This two-step approach may be attractive because it may separate the computationally intensive inversion step from the noise-focused processing step. However, this two-step approach may be sub-optimal as prior knowledge of subsurface resistivities may not be taken fully into account in determination estimates of electromagnetic earth responses in the processing step, and the inversion step does not take into account detailed knowledge about the noise and its properties.